Technical Field
The present disclosure relates to an integrated piezoelectric sensor for detecting in-plane forces, such as shocks, accelerations, uniaxial forces and accelerations, and rotational forces. In particular, the disclosure regards a sensor obtained using semiconductor technology, typically with the technology for the manufacture of MEMS devices, for detecting forces acting in the plane of the sensor.
Description of the Related Art
As is known, when piezoelectric materials are subjected to physical stresses and undergo deformation, they are biased, thus generating a potential difference across them and generating electrical charges. By connecting these materials to an external circuit, a piezoelectric current correlated to the forces applied is thus obtained.
The above phenomenon has been studied for years and is exploited in order to provide sensors where a sensing structure, typically a cantilever beam or cantilever having at least one piezoelectric region, undergoes deformation following upon mechanical stresses and generates an electric current. By connecting the sensing structure to a measurement circuit, such as an amperometer and a processing stage, the latter may detect the charge or potential difference and determine the force acting on the cantilever.
In this way, a piezoelectric sensor is able to measure forces such as linear and rotational forces, for example accelerations, shocks, etc.
The geometrical dimensions, the properties of the materials, and in general the entire design of the sensing structure of the sensor are generally optimized according to the physical quantity to be detected.
For instance, for a shock sensor, it is possible to use a sensing structure 1 as shown in FIG. 1. Here, a piezoelectric sensor 1 comprises a cantilever 2 carrying a piezoelectric layer 3, for example a PZT (lead zirconate titanate) crystal. The cantilever 2 is constrained in 7 and has one free end 8. In case of external forces 4 acting on the cantilever 2, these cause curving and upward or downward bending of the cantilever 2, as indicated by the arrow 5. This bending causes a deformation of the free end 8 of the cantilever 2 and generation of a stress that may be detected via a suitable measurement circuit.
The piezoelectric sensor 1 of FIG. 1 is suitable for detecting deformations due to forces or stresses acting in a perpendicular direction to the lying plane of the cantilever 2, so-called “out-of-plane direction”. Thus, in the example shown, wherein the cantilever 2 extends to a first approximation in the plane XY, the piezoelectric sensor 1 is able to detect forces and stresses causing movement of the free end of the cantilever 2 in direction Z.
The piezoelectric sensor 1 is not, however, able to detect the action of forces and stresses acting in the plane XY. In order to detect these forces, the piezoelectric sensor 1 is rotated through 90° in such a way that the cantilever 2 extends parallel to a plane passing through the axis Z.
This, however, causes production of the sensing structure to be decidedly complex, since manufacturing and assembly are complex and entail higher costs, the overall dimensions of the sensing structure are greater, and the sensing structure has a lower precision than the in-plane sensing structure.
Other known solutions envisage embedding, in the structure of the cantilever, layers of piezoelectric material that extend according to a lying plane transverse to the plane XY, for example at 45° with respect to the sensor plane. These solutions are, however, particularly complex from the manufacturing standpoint, and thus costly. They may not thus be used in all low-cost applications.